Reduction of interference in carrier systems



Jan. 1933. v. P. THORP REDUCTION OF INTERFERENGE IN CARRIER SYSTEMSFiled June 18, 1952 INVENToR Vj? WMI/y0' ATTORNEY Patented Jan. 10, 1933NETE@ STATES regatta VAUGHN P. THOR-P, OF RIVER EDGE, NEW JERSEY,

ASSGNOR TO AMERICAN REDUCTION 0F NTERFERENCE IN CAR JIER SYSTEMS cApplication filed June 18,

This invention relates to A. C. telegraph systems, and particularly tomeans for reducing the effect of extraneous interference upon thereceiving apparatus of such systems.

5 In the carrier telegraph systems of the types that are well known inthe art, the carrier currents received from the transmitting medium arerectified, and the rectified cur- M rents used to operate a polarrelay.. This l relay normally has a biasing Winding through which acurrent flows steadily of such a value that it neutralizes approximatelyone-half the effect of the normal operating current when both currentsare flowing. This causes the armature to be moved from one contact tothe other with equal force in either direct-ion, as the operatingcurrent is alternately on and off. Tnterfering currents set up in thetransmitting medium by sources eX- traneeus to the syste-m itself oftencause false operation of the relay which in turn causes false signals tobe received on the receiving mechanism. Lightning hits are usually ofvery short 3 duration but of large magnitude, this latter effectcausingsustained oscillation of the receiving selecting circuits over a periodcomparable to a dot length in high speed signaling. Current limitingdevices ahead of the 0 tuned circuits have been used to some extent toreduce the shock excitation of these circuits and such a. device, inconjunction with other arrangements, forms a part of this invention. For best protection against interference it has been found fromexperience that the current limiting device should have al drooping`characteristic; that is, when the amplitude of the incoming wave exceedsa predetermined value the output of the device is reduced to only asmall fraction of its previous value. For this reason the incomingsignaling currents are also eliminated for the duration of theinterfering hit, sometimes causing a false spacing signal. Thisinvention provides an arrangement which eliminates the latterundesirable effect.

This invention will be clearly understood from the following descriptionwhen read in connection with the accompanying drawing, the figure ofwhich illustrates schematically 1932. serial No. 618,057.

a circuit arrangement embodying the invention.

In* the drawing the line Lk represents a, transmission circuit or mediumover or through which a plurality of carrier frequencies such as f1 andf2 may be transmitted from the carrier transmitting apparatus 1 to thecarrier receiving apparatus connected with the other end of the saidtransmission circuit. At the receiving station the apparatus is renderedselective to the incoming group of receiving frequencies by thereceiving filter RF. After passing through the interference suppressionapparatus, later to be described, the carrier signaling frequencies aredirected to their respective detectors by the tuned circuits A and Bwhich are selective to frequency bands based upon the carrierfrequencies ,f1 and f2 respectively.v wWhile a tuned circuit has beenshown for the selection of a particular frequency band, such showing ispurely illustrative, since other welllrnown selective devices, such asfilters, may be used. The output of the tuned circuit is connected toamplifier 6 and the output of amplifier 6 is connected to the gridcircuit of detector 7. The plate circuit of this detector, whichincludes a source of plate voltage 35, is connected with the winding 12of polar relay 27. T he grid potential of this detector is maintainedsufficiently negative to prevent the flow of plate current when nosignal is heilig received. Another winding 13 of this relay is connectedto battery 35 and ground. This winding is so poled and the magnitude ofthe current so adjusted as to tend to hold the armature 16 against itsspacing Contact. The contacts of relay 27 are connected with sources ofpotential 20 and 21 which have opposite poles connected to ground. Thearmature 16 is connected by conductor 17 to the relay 18 of thetelegraph substation. Armature 16 also has connected to it a leakcircuit 28 which includes the winding of a polar relay 11. The armature22 of this relay connects to the grid circuit of the signal channeldetector 7 through resistance 29. When no lightning interference orcarrier signal is being received, the negative potential o-n the grid ofdetector 7 is the same as the potential of the battery 26. Since thepurpose of the polar relay l1 and the manner in which it cooperates withother apparatus Will be apparent from the description of the mode ofoperation of the system, it is believed that further description of itis unnecessary at this point.

The lightning suppression arrangements will now be described. rllhecurrents of the received signal frequencies, and such eX- traneonsinterfering currents as may be present, pass through the receivingfilter RF and encounter amplifiers 2 and 3 in parallel. Amplifier 2 iscoupled to the input of amplilier 4 by the transformer 30. The output ofamplilier 4 connected to the signal channel receiving selectingcircuit-s by transformer 3Q. Amplilier 3 is coupled to full-Wavedetector 5 by transformer 3l. The source of grid potential Bi and thesource of plate potential B1 of detector 5 are so usted that, for normalamplitude of the combined currents of the incoming carrier signalingfrequencies and iviti no interference present, the plate current ofdetector 5 Will be Zero. Under this condition the grid potential ofamplilier 4 Will equal the potential of battery B2, and amplifier 4 willfunction as an ordinary amplilier having a constant gain. When aninterfering current of large amplitude is impressed. on the normalcombined signal currents, it v-:ill cause plate current to flow indetector 5. rlhis plate current, supplied by battery B1, flows from thepositive terminal of this battery to the detector plates, then to thefilament, to ground, from ground to resistance Rl, through R1 to thenegative terminal of battery B1. The IR drop across B1 increases thenegative grid potential applied to ampliliers 4 and 9. In the case ofamplilier 4, this increase in negative potential on the grids increasesthe plat -to-filament impedance. which reduces considerably the overallgain of the amplifier which momentarily eliminates both interference andmarking signals. The increase in negative grid potential of amplilier 9reduces momentarily the plate current in amplifier 9. The plate circuitof amplifier 9 includes the battery BU and one Winding of transformer25. The sudden reduction in plate current through this Winding inducesvoltage in the secondary of transformer 25 to which is connected afull-Wave rectifier of any Wellknoivn type such as the copper-oxidetype. This rectified current fiovvs through resistance R2 and the llldrop thereby produced is used to vary the potential applied to the gridsof the signal channel detectors While a lightning hit is coming in. Thepolar relay ll, operating from the armature of the signal channelreceiving relay, will follow the incoming telegraph signals. ltsarmature Will rest on the marking contact 23 When a marking signal isbeing received and on the spacing contact 24 when a spacing signal isbeing received. Thus the change in negative grid potential of detector7, that is, Whether the negative potential is increased or decreasedWhen a lightning hit comes in, is determined by the position of thearmature of polar relay 1l which is determined by the incoming telegraphsignal, that is, Whether it is marking or spacing. Resistances 33 and 34are connected between the armature and spacing and armature and markingcontacts respectively, of relay 1l, in order to maintain thc gridpotential of detector 7 during the travel time of the armature of relay11.

The operation of the system may be briefly summarized as follows: Whenno interference is coming in, the received group of carrier frequenciesis selected by the receiving iilter RF, amplified by ampliers 2 and 4and passed on through transformer 32 to the individual signal channelselecting circuits. The frequencies are ultimately rectified and used tooperate the respective receiving relays. The current of the receivedgroup Will also be amplified by amplifier 3 and impressed on detector 5,but no change occurs in detector 5 or amplifier 9 since the negativepotential of source B4 applied to the grids of detector 5 is somewhathigher than the maximum peak of the combined incoming signals; hence, noplate current floivs in the plate circuit of detector 5. Since there isno plate current flowing in resistance R1, there is no change ofpotential of the grids of amplifiers 4 or 9. rllhere Will be no voltageapplied to rectifier 10; hence, no rectified current will flow inresist-ance R2 and the steady grid potential applied to the grid ofdetector 7 will be that of battery 26. This condition will existregardless of the position of the armature of polar relay l1 so long asno lightning hit is coming in. Assume noW that a marking signal is beingreceived on channel l and a lightning hit of large magnitude occurs nearthe transmission medium L. The amplitude of the received Wave will besutiiciently large to overcome the potential of B.L applied to the gridsof detector 5 and plate current will flow through R1. The resulting IRdrop across R1 will increase the negative potential applied to the gridsof amplifiers 4 and 9. In the case of amplifier 4, its gain will begreatly reduced as explained previously, thus preventing the lightninghit from passing through to the signal channel tuned circuit and settingup sustained oscillations. The reduction in gain of amplifier 4 Willalso momentarily eliminate the signal current being received and,Without other features described herein, a false spacing signal might bereceived. This is prevented as follovvs: The change in potential of thegrid of ampliiier 9 will cause a change in plate current of that tube.The voltage setup across the secondary of transformer 25 Will causerectified current to flow in resistance R2 in the direction indicated bythe arrow. The IR drop across the lower half of resistance R2 willsubtract from the potential of the battery 26, making the potentialapplied to the grid of detector 7 less negative, which will maintain theplate current in detector 7 and hence maintain the marking signal.

Assume now that a spacing signal is being received on channel l. Theaction of the suppression arrangements will be described below for thisspacing or no current condition. Since no carrier current is beingapplied to the grid of detector 7, no plate currentis flowing in thattube because the negative potential provided by battery 26 issufficient-ly large to prevent flow of plate current. The biasingcurrent through winding 13 of the receiving polar relay will hold thearmature of the relay on its spacing contact. Polar relay ll will haveits armature resting on the spacing contact 24. I/Vhen a lightninof hitarrives, the action of amplifier 4, amplifier 9, detector 5, andrectifier l will be identical to that described for the case when amarking sional is being received. During the period while the incominginterfering wave is building up to the value where detector starts tofunction to reduce the gain of amplifier 4, some current will passthrough amplifier 4 to the signal channel selecting circuit. Some delaywill be encountered while the signal is passing through the selectingcircuit of channel l. Meanwhile, t-he rectified current flowing throughRgwill produce an IR drop which will add to the negative potential ofbattery 26, since "the armature of relay ll is on its spacing contact.This added negative potential applied to the grid of detector 7 willmaintain the spacing signal.

The operaton of other channels, such as channel 2, will be similar inall respects to that of channel l and therefore need not be described.

It should be noted that the neutralizing arrangement supplied inaddition to the current limiting circuit is a fast operating' device. Inprevious neutralizing devices, tuned circuits have been used to select anarrow band of the interfering frequencies, rectifying these currentsand passing the rectified currents through a winding of the receivingrelay in such a way as to oppose the interfering pulse which occurs inthe regular signal channel detector. In the latter arrangement theinterfering pulse is usually prolonged by the oscillation of the tunedcircuits in such a way that the receiving relay armature is notpermitted to change its position during this period of oscillation whichmay result in serious distortion of the telegraph signal. In thearrangement disclosed herein, the signal channel tuned circuit isprotected against shock excitation by the action of the current limitingportion of the circuit, and a holding or neutralizing pulse is producedwithout the use of tuned circuits, this latter feature producing aneutralizing pulse which lasts only for the duration of the interferinglightning hit which is ordinarily extremely short.

The neutralizing pulse is unidirectional, as the direction of thecurrent through the resistance Iig is determined by the full-wave fromthe spirit of the invention as delined in the following claims.

What is claimed is:

l. In a high frequency receiving circuit, the combination with anamplifier upon which high frequency signaling currents and'vinterference curr-ents may be impressed, the said amplifier beingadjusted to limit the amplitude of the current wave transmittedtherethrough, of a detector effectively connected Vto said amplifier, asignal receiving device connected to the output of said detector,another detector upon which the said signaling and interference currentsmay be impressed, the said other detector being connected to saidamplifier to control the limits ofthe latter, and means also controlledby said other detector, to regulate the action of said first mentioneddetector whenever vthe said amplifier is functioning to limit thecurrent therethrough.

2. In a high frequency receiving circuit,

-the combinationvvith a main receiving path, upon which high frequencysignaling cur-v rents land interference currents may be impressed,containing a current limiting amplifier, a detector effectivelyconnected to the out-put of said amplifier, a signal receiving device,responsive to marking and spacing impulses, connected to said detector,a control path, bridged across said main path, con- ,f

taining another detector having its output connect-ed to the amplifierof said main path to adj ust thesetting thereof Whenever the current inthe control path reaches a fixed limit and means connected between saidother de- 'tector and said first mentioned detector to prevent falseoperation of said signal receiving device by the functioning of saidamplilier during the receipt of signals.

3. In a high frequency telegraph receiving system, the combination witha vacuum tube amplier upon which signal currents and interferencecurrents may be impressed, of a vacuum tube detector having its inputcircuit effectively connected to the output circuit of "roo TLID

said amplifier, a pola-r relay connected to the output circuit of saiddetector, a second vacuum tube detector upon which also the saidcurrents may be impressed, the said second detector being so adjustedthat the current in the plate circuit thereof is substantially zero,

a resistance connected to the said plate circuit, means to apply to thegrid circuit of said amplifier the negative voltage due to the flow ofcurrent through said resistance, another vacuum tube amplifier havingits grid connected to said resistance, means effectively connected tothe plate circuit of said other amplifier to rectify the currentvariations in said plate circuit, and another polar relay, controlled bythe said first mentioned polar relay, to apply the rectified impulses ofsaid other amplifier to the grid of said first mentioned detector insuch manner as to prevent false operation of said first mentioned polarrelay When the current limiting amplifier operates during the receipt ofmarking and spacing impulses.

4. In a high frequency telegraph receiving system, the combination witha detector of a polar relay for receiving signals, means connected tothe input of said detector t0 limit the amplitude of the currentsapplied thereto and means also connected to said detector to prevent thefalse operation of the polar relay by the functioning of the currentlimiting means during the receipt of signals.

5. In a high frequency telegraph receiving system in Which marks andspaces are represented by the presence and absence respectively of atrain of Waves, the combination with a. vacuum tube detector to rectifythe received Waves of a polar relay having an armature movable from onecontact to another whenever the received signal changes in character, avacuum tube amplifier functioning as a current limiting device connectedbetween the source of high frequency signals and the said detector, arectifier, responsive only to currents of predetermined values to applyan increasing negative voltage to the grid of the said amplifier tolimit the current passed thereby, and means, also controlled by saidrectifier, to apply a voltage to the grid of said detector of suchmagnitude'and polarity as to keep the armature of the receiving relay onthat contact upon which it Was resting when the current limiting actionbegan.

In testimony whereof, I have signed my name to this specification this15th day of June, 1932.

VAUGHN P. TI-IORP.

